I Spent $3,200 on Johns Manville Foam Board Before Learning This One Rule About Temp Control Mineral Wool

If you're using Johns Manville Temp Control mineral wool and foam board together on the same project, install the foam board first. That single piece of advice would've saved me about $600 on a job last year. Here's why I say that with such certainty, and the other mistakes I made along the way so you don't have to repeat them.

The $600 Mistake (And Why I'm So Sure About the Order)

I install insulation for a living. In September 2022, I took on a commercial retrofit—a small office space, about 1,200 square feet. The spec called for both Johns Manville Temp Control mineral wool in the interior walls (for sound) and JM foam board on the exterior. I'd done plenty of jobs with one or the other, but never both together on the same wall assembly.

Here's what I did wrong: I installed all the mineral wool first. It seemed logical—fill the cavity, then cover it with the foam board for the continuous insulation layer. But that logic failed me. The mineral wool compresses easily. When I tried to fit the foam board over it, the mineral wool pushed back against the fasteners. The foam board wouldn't sit flush. I ended up having to pull sections of the mineral wool out, trim them down, and try again. It took an extra day and a half, plus the cost of new fasteners and some damaged foam board.

The lesson: install the rigid foam board first against the exterior sheathing. Then fit the mineral wool into the cavity from the interior side. The foam board gives you a solid, uncompressed backing. The mineral wool fits against it perfectly. 5 minutes of planning upfront beats 5 days of correction.

Why I Specifically Use Johns Manville (And Where It Can Backfire)

Look, I'm not a brand loyalist for the sake of it. I've used Owens Corning and CertainTeed plenty. But for mineral wool, I've settled on Johns Manville Temp Control for a few reasons:

• Its density is consistent. I've unrolled batches where the thickness varies by less than 1/8 inch. That matters when you're trying to get a friction-fit to stay put without sagging over time.
• The facing on their foam board is more tear-resistant than some competitors I've tried. Less of a headache when you're wrestling it into place on a cold morning.

That said, I've also made the mistake of assuming all JM products are interchangeable. They're not. Temp Control mineral wool is designed for thermal and acoustic performance in walls and ceilings. It's not the same as their pipe insulation products, which have a different density and temperature rating. Don't swap them. I learned that the hard way on a small job where I used scraps of Temp Control on a hot water pipe. It sagged within six months.

The Glass Bottles Incident (And a Weird Tempered Glass Lesson)

This might sound like a tangent, but stick with me—it's relevant to understanding what should and shouldn't go near your insulation.

A while back, I was renovating a space that had previously been a small glass studio. There were boxes of used glass bottles and broken glassware left behind. I figured I'd recycle them. No big deal, right? Except I was also installing insulation nearby and had some scraps of foam board and mineral wool stacked against the wall where I was sorting the glass.

One of the bottles—a thick, green wine bottle—rolled off the table and landed right on the edge of a piece of foam board. The board crunched. It wasn't a clean cut; it was a compression fracture that left the board compromised. The bottle, of course, was fine. Glass bottles are tough. But it got me thinking about how different types of glass handle impact. Regular glass bottles shatter explosively. Tempered glass, like you'd find in a shower door or a car window, crumbles into small cubes. But a bottle? It's either annealed or, rarely, tempered. The point is: don't drop heavy glass objects on your insulation. Obvious? Sure. But it's exactly the kind of dumb thing you don't think about until you've done it.

More practically: if you're working in a space where there's existing tempered glass—like in a storefront or a window near your install area—be mindful of how you handle and store your insulation boards. A stray piece of mineral wool won't hurt it, but a swinging foam board can scratch or chip the edge. And a chipped edge on tempered glass means the whole pane needs replacing. I've seen it happen. It's not cheap.

The Hidden Problem: Wallpaper Glue

Here's another one that cost me a day of scraping. I was renovating an older building, and the previous tenant had left walls covered in wallpaper. The client wanted new insulation installed. I figured I'd just pull the wallpaper off and stick the mineral wool in. Big mistake.

Old wallpaper glue—especially the stuff from the 70s and 80s—can leave a thick, gummy residue on the drywall or plaster. If you install insulation over that residue, you've created a moisture trap. The glue holds moisture against the wallboard. Over time, that can lead to mold or delamination of the wall surface. Plus, the adhesive in the paper can react with the facing on some foam boards, causing it to peel.

So, here's my checklist for removing wallpaper glue before insulating:

1. Test a small area first. Use a spray bottle to wet the glue and see if it softens. Some glues dissolve with water; others need a chemical stripper.
2. Scrape thoroughly. A 4-inch drywall knife works best. Don't rush it. Every bit of residue you leave is a potential problem.
3. Wash the wall. After scraping, wipe the surface with a damp sponge to remove the last of the glue. Let it dry completely—at least 24 hours—before installing insulation.
4. Consider a primer. If the glue left a stubborn stain or residue, use a stain-blocking primer (not a standard latex) before insulating. It seals the surface and prevents any reaction between the glue and the insulation facing.

I skipped step 3 on that job. Three months later, the client called about a musty smell. We opened the wall and found mold on the back of the drywall. The glue had trapped moisture from a small leak in the roof. The fix cost $2,000. A 30-minute cleaning step would have prevented it.

So, Here's My Honest Takeaway

Johns Manville makes good products. Temp Control mineral wool and their foam board are solid choices for most wall assemblies. But they're not magic. They need to be installed correctly, in the right order, on a clean surface.

The rules I follow now:

• Foam board goes in first, mineral wool second.
• Remove all old wallpaper glue before insulating. Don't cut corners.
• Be careful what you store near your insulation. Heavy glass bottles, sharp tools, or anything that can crush or tear the board is a risk.
• If you're working near tempered glass, protect it from accidental impacts with swinging insulation boards.

This advice won't apply to every job. If you're insulating a new-build with pristine walls and no glass hazards, you can skip some of these steps. But for renovations, retrofits, and messy demo-to-finished projects? These are the things I wish someone had told me before I lost $3,200 in mistakes.

Dodged a bullet on a recent job when I remembered the wallpaper glue lesson. Almost skipped the wash step to save time. Would have cost me big.